The present invention relates to a multi-beam scanning optical system.
In laser recording applications such as laser printers, scanning optical systems are employed in which the recording surface is scanned with a laser beam from a semiconductor laser after it is deflected by such a device as a rotating polygonal mirror or a hologram disk. In order to increase the recording speed of such apparatus, faster scanning may be effected by speeding up the rotation of the light deflector such as a rotating polygonal mirror or hologram disk. However, realization of higher rotating speeds requires a sophisticated and expensive bearings such as pneumatic or magnetic bearings and this leads to an increase in the cost of the apparatus.
To avoid this problem, a method has been developed that is capable of achieving a substantial increase in the scanning speed without increasing the rotational speed of the light deflector. In this method, a plurality of laser beams are simultaneously directed into the light deflector so as to achieve simultaneous scanning with a plurality of scanning lines.
A conventional optical system that employs this "multi-beam scanning" method is depected in FIG. 1. As shown in FIG. 1, the system comprises semiconductor lasers 1 and 2 as light sources, collimator lenses 3 and 4 that convert the radiation from the semiconductor lasers 1 and 2 into parallel rays of light, a polarized beam splitter 7 that superimposes the collimated laser beams 5 and 6, a rotating polygonal mirror 8 that allows the laser beams from the beam splitter 7 to be deflected with respect to a scanning surface 9, and an f.theta. lens 10 that receives the beams from the polygonal mirror 8 and focuses them to form spots of light on the scanning surface 9.
In this system, in order that a plurality of linearly polarized beams from the semiconductor lasers 1 and 2 can be superposed by the beam splitter 7, the two semiconductor lasers 1 and 2 are required to create S and P waves, respectively, with respect to the beam splitter 7. For this purpose, the semiconductor lasers 1 and 2 are positioned in such a manner that they are offset from each other by an azimuth angle of 90 degrees with respect to optical axes of the beams from the respective lasers. That is, the semiconductor lasers 1 and 2 are positioned such that the direction of the junction plane of the semiconductor laser 1 is perpendicular to the direction of the junction plane of the semiconductor laser 2. As a result, a direction of polarization of the beam produced by the semiconductor laser 1 is perpendicular to the direction of polarization of the beam from the laser 2, since the direction of polarization of the beam produced by a semiconductor laser is parallel with a direction of a junction plane of the semiconductor laser. For example, as shown in FIG. 1, the semiconductor laser 1 is placed such that the junction plane thereof is placed in parallel with the surface of the drawing. The semiconductor laser 2 is placed with its junction plane being perpendicular to the surface of the drawing. As a result, the vector of polarization of beam from the laser 1 is parallel with the surface of drawing, as indicated by arrow A in FIG. 1. On the other hand, the vector of polarization of beam from the laser 2 is perpendicular to the surface of the drawing, as indicated by arrow B.
The conventional system described above operates as follows. Radiation from the semiconductor lasers 1 and 2 is collimated by associated collimator lenses 3 and 4 to produce parallel rays of light which are then directed into the polarized beam splitter 7. Since the two laser beams are polarized perpendicular to each other, virtually all of the beam from the laser 1 is transmitted through the splitter 7 while the beam from the laser 2 is reflected almost completely. As a result, substantially no optical loss occurs in the two beams as they are superimposed in the beam splitter 7 to become aligned in substantially the same direction. The superimposed or aligned beams are then deflected by the rotational polygonal mirror 8 and passed through the f.theta. lens 10 to scan the scanning surface 9.
As described above, in the conventional multi-beam scanning optical system, the two semiconductor lasers which are intended to produce S and P waves are offset from each other by an azimuth angle of 90 degrees with respect to optical axes of the beam from the laser. The angle of spread of radiation from a semiconductor laser is different from each other in two directions, i.e., in the direction of polarization of the radiation and in the direction perpendicular to it, due to the emission distribution characteristics of the semiconductor laser itself. Therefore, in the conventional system, the beam spots 11 and 12 formed on the scanning surface by the beams from the semiconductor lasers 1 and 2 are different from each other in their shapes, as shown in FIG. 4. In this case, since two lines are scanned simultaneously with the two laser beam spots, the scanning speed can be increased. However, the two laser beams used for the simultaneous two-line scanning have angles of spread of radiation different from each other. Therefore, the resolution of the respective beams are different from each other. As a result, the above-described conventional multi-beam scanning optical system is unable to obtain a uniform or even picture.
In order to make the resolutions of the two beams equal to each other, the beams may be converted into beams having a circular cross section by the collimator lenses so that the beam spots 11 and 12 formed on the scanning surface may be in circular shape, as shown in FIG. 5. In this case, in a sub-scanning direction, i.e., in a laser spot scanning direction, data is recorded by elliptic beam spot. On the other hand, in a main scanning direction, i.e., in a data recording medium scanning direction, data is recorded by a circular beam spot. The conventional system using circularly shaped beam spots has a problem that an edge portion formed by the beam spots on the scanning surface is not made smooth, as shown in FIG. 5.
Therefore, it is preferable that the multi-beam scanning is performed such that the longitudinal direction 13 and 14 of the beam spots 11 and 12 coincide with the main scanning direction, i.e., the scanning surface moving direction, as shown in FIG. 6. However, for this purpose, in the conventional multi-beam scanning system, two types of collimator lenses, one for P wave and the other for S wave, are required, and this necessitates the use of more than one type of optical lenses.